Along with the recent remarkable development of semiconductor element manufacturing techniques, the progress of micropatterning techniques is also prominent. As an optical processing technique, reduction projection exposure apparatuses, generally called steppers, having submicron resolutions, are mainstream. For higher resolutions, a larger numerical aperture (NA) of an optical system and a shorter exposure wavelength are being realized.
As the exposure wavelength decreases, the exposure light source is shifting from high-pressure mercury lamps of g-line and i-line to KrF and ArF excimer lasers.
As the resolution of a projection pattern increases, high precision is also demanded for relative alignment between a wafer and a mask (reticle) in a projection exposure apparatus. The projection exposure apparatus needs to function not only has a high- resolution exposure apparatus, but also as a high-precision position detection apparatus.
For this purpose, high performance is required for a position detection apparatus, also called an alignment scope, for detecting an alignment mark on a wafer, or the like.
As the form of the alignment scope, there are roughly proposed two methods. One form of an alignment scope is a so-called off-axis alignment detection system (Off-Axis Auto Alignment; to be referred to as an “OA” hereinafter), which is separately disposed without the mediacy of a projection exposure optical system and optically detects an alignment mark.
One of the conventional alignment methods using an i-line exposure apparatus is a method of detecting an alignment mark on a wafer by using the alignment wavelength of non-exposure light via a projection optical system called TTL-AA (Through The Lens Auto Alignment).
Currently, in either detection system, a method, in which an image (image data) of an alignment mark to be observed is converted to an electrical signal by a photoelectric conversion element, and the position of the alignment mark is calculated on the basis of the electrical signal, is become mainstream in view of the precision and flexibility of various semiconductor processes.
A projection exposure optical system having a conventional OA detection system will be described with reference to the schematic view in FIG. 4.
Light IL emitted by an exposure illumination optical system 1 including an exposure light source (as the light source, a mercury lamp, a KrF excimer laser, an ArF excimer laser, or the like) illuminates a mask (reticle) 2 having a pattern. At this time, the reticle 2 is aligned in advance by reticle holders 12 and 12′ such that an alignment detection system 11 above (or below) the reticle 2 makes the center of the reticle pattern coincide with an optical axis AX of a projection exposure optical system 3.
The light having passed through the reticle pattern transfers the image of the reticle pattern onto a wafer 6 held on a wafer stage 8 at a predetermined magnification via the projection exposure optical system 3. Note that an exposure apparatus for irradiating the reticle 2 with irradiation light from above the reticle 2 and sequentially exposing the wafer 6 to reticle patterns at a fixed position via the projection exposure optical system 3 is called a stepper. An exposure apparatus for relatively driving the reticle 2 and wafer 6 (the driving amount of the reticle 2 is the product of the driving amount of the wafer 6 and a projection magnification) is called a scanner (scanning exposure apparatus).
A kind of wafer 6 is called a second wafer already bearing a pattern. To form the next pattern on this wafer, the position of the wafer must be detected. This position detection method includes the TTL-AA method and OA detection method described above.
An alignment scheme having an OA detection system will be explained based on FIG. 4.
As shown in FIG. 4, an OA detection system 4 is arranged separately from the projection exposure optical system 3. The wafer stage 8 is driven based on an interferometer 9 capable of measuring a lateral distance. The wafer 6 is aligned in the observation region of the OA detection system 4. The OA detection system 4 detects the position of an alignment mark formed on the wafer 6 aligned based on the interferometer 9, thereby obtaining layout information of chips (elements) formed on the wafer 6.
The wafer stage 8 is driven to the exposure region of the projection optical system 3 (transfer region of the reticle) on the basis of the chip (element) layout information. Then, the wafer 6 is sequentially exposed.
A focus detection system 5 for measuring the focus direction of the projection optical system 3 is generally located in the exposure region of the projection exposure optical system 3. In the focus detection system 5, a slit pattern 503 is illuminated via an illumination lens 502 with light emitted by an illumination light source 501. The light having passed through the slit pattern 503 forms a slit pattern on the wafer 6 via an illumination optical system 504 and mirror 505.
The slit pattern projected on the wafer 6 is reflected by the surface of the wafer 65, and enters a mirror 506 and detection optical system 507 arranged on a side opposite to the focus detection system 5. The detection optical system 507 forms the slit image of the wafer 6 on a photoelectric conversion element 508 again. As the wafer 6 vertically moves, the slit image on the photoelectric conversion element 508 moves. From this moving amount, the distance of the wafer 6 in the focus direction can be measured. A plurality of such slits (points on 6 the wafer 6) are prepared, and the focus position of each slit is detected (the points on the wafer 6 are measured). As a result, the inclination of the wafer 6 with respect to the image plane of the reticle image of the projection exposure optical system 3 can be measured.
In the projection exposure apparatus as described above, alignment marks AM formed on an actual process wafer as the wafer 6 have different longitudinal and lateral structures. In addition, an alignment detection system capable of making an illumination condition (to be referred to as an “illumination mode” hereinafter) variable (e.g., wavelength selection and variable NA) is adopted so as to detect these various alignment marks AM at high precision.
Although a description of the TTL-AA method will be omitted, a wafer is basically observed by the OA detection system via the projection exposure optical system 3 in the TTL-AA method. Various alignment marks can be detected while varying the illumination condition.
However, when an alignment mark on a wafer is observed to detect the position by the above-mentioned alignment scope, a resulting alignment detection signal changes, depending on the structures (longitudinal and lateral structures) and type of the alignment mark. Accordingly, optimization of the illumination mode is required to detect such a signal at high precision. For this reason, it is necessary for the device side to select and recommend for the user the optimum alignment structure and illumination mode for various wafer processes.
Conventionally, for these various processes, the optimum illumination mode is selected only in consideration of the contrast of a resulting alignment detection signal waveform. Consequently, in some cases, mismatching between te optimum illumination mode and the final alignment precision may occur.
In addition, when the performance of an alignment scope is to be evaluated, it is judged from its repeatability (e.g., 3σ or range) obtained by continually measuring an alignment mark formed on a wafer stage (including the arrangement on a wafer). However, in this case, the desired performance of an alignment detection system cannot be evaluated satisfactorily due to the influence of the stability/instability of the wafer stage itself (e.g., the stability/instability of an interferometer).
Moreover, linearity (the linearity of measurement values in driving an alignment mark in the measurement direction), one of the characteristics of an alignment detection system, is not regulated, and besides, a shift from the linearity (to be referred to as a “linearity residue” hereinafter) occurs. The cause has not been specified, and a method for adjusting such shift has not been established.
When the measurement precision (linearity) of the position of an alignment mark in the measurement direction degrades, an alignment mark needs to be fed to a predetermined position with respect to the alignment detection system, thereby decreasing the throughput.